Diagnosing a timeline that has stopped playing

Slow is a symptom with several causes

Playback drops to eight frames a second. The fan runs. You conclude the machine is inadequate. Usually one specific thing is responsible, and the diagnosis is quick if you know the order to check.

The order to check

1. Are you playing proxies? The first block covered this. Long-GOP source files are expensive to decode, and a proxy removes the problem entirely. Check that proxy media exists and that the toggle to use it is on — these are two separate settings and forgetting the second is extremely common.

2. Is a single effect responsible? Disable all effects — Resolve has a bypass colour grades and fusion effects button; most editors have an equivalent — and play. If it is fine, re-enable one at a time. Nine times in ten the culprit is one node, one effect, or one nested timeline.

The usual suspects, roughly in order of cost: noise reduction, optical flow retiming, motion blur, large blurs, stabilisation before it is cached, any Fusion composition, and a high-resolution PNG scaled down in real time.

3. Is it the scopes? Scopes recompute every frame and can cost a third of your playback performance. Close them when you are not reading them.

4. Is playback resolution set to full? Most editors let you play at half or quarter resolution while keeping full for export. Resolve has Timeline Proxy Mode in the Playback menu. This is a one-click improvement that many people never find.

5. Is something rendering in the background? Cache generation, proxy generation, an export. Check before concluding anything.

6. Is the disk the bottleneck? 4K ProRes runs at roughly 700 Mbit/s, which a slow external drive over a slow connection will not sustain, particularly with two streams for a multicam. If the machine is idle and playback still stutters, suspect the drive.

Render cache

The general solution for the effects problem: let the computer render the expensive sections in the background, and play the rendered result.

Resolve calls this Smart Cache, with modes for Smart, User and Off, plus separate cache settings for Fusion output and for colour output. Smart mode automatically caches anything it considers expensive. User mode caches only what you flag.

On a slower machine, the working method is: set cache to Smart, leave the timeline for a minute after each big change, and play. On a faster one, User mode gives you more control over disk use.

The cache goes to the location you set in project settings — point it at your working drive, not the system disk.

Nested timelines and compound clips

A compound clip or nested timeline is convenient and expensive: the editor renders the nest, then applies whatever is on it, which is two passes. A timeline with six nested compositions is doing six times the work.

Where a nest exists only for tidiness, flatten it. Where it is load-bearing, cache it.

GPU and CPU

Most modern editors do colour, effects and scaling on the GPU and decoding on either. What this means practically:

• Resolve wants a real GPU with 4 GB or more of video memory, and more for 4K work. On integrated graphics it runs and it is unpleasant.
• Hardware decoding for H.264 and H.265 is usually a checkbox in preferences. Turn it on. On some hardware and some codecs it is unreliable; if playback is glitchy rather than merely slow, try turning it off.
• RAM matters for cache. 16 GB is comfortable for 1080p; 32 GB for 4K with effects.

If Resolve is a fight on your machine, Shotcut and Kdenlive are genuinely lighter and will edit 1080p comfortably on hardware that Resolve struggles with. That is a real option rather than a consolation prize, and the earlier lessons on cutting, structure and sound apply identically in them.

The optimisation that costs nothing

Work at 1080p even when delivering 4K, and finish at 4K only at the end. The whole edit — structure, cutting, sound — is unaffected by resolution, and a 1080p timeline is roughly four times cheaper to play.

In Resolve you can change the timeline resolution at the end, though as the setup lesson warned, this re-frames every clip whose scaling was set relative to the old frame. The safer version is to work on proxies at full timeline resolution, which achieves the same thing without changing any geometry.

Today

Bypass all effects on your slowest timeline and play it. Then re-enable them one at a time until it stutters. You will find the culprit in about three minutes.