ID 420306 - [C] Loading a sequence revision with a lot of frames locks up the UI

Problem summary
Flix Client is very slow at loading thumbnails and becomes unresponsive after the local cache is cleared, especially if the opened Sequence has a lot of frames.

Customer reported version
Flix 6.2.1

Customer reported platform
CentOs7

Steps to reproduce

1) Clear your Flix Client cache​.

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2) Open a Sequence containing a lot of frames (not panels), for example over 1000. Flix will take a very long time to load the panels (~1-2 minutes), or sometimes fail completely and display only white panels until the UI is reloaded (cmd/Ctrl+shift+R). Client is sometimes not responsive while the panels are being loaded.

Expected behaviour
Sequences should not take so long to load

Actual behaviour
Client becomes unresponsive and panels take a long time to load; sometimes panels will appear to have loaded but all appear blank.

Workaround
Reloading the Client usually loads panels instantly, or after a much shorter time (5-10 seconds).

Reproduced by support

This bug has been reproduced in:
6.2.2 - Windows 10

6.2.1 - Windows 10 - macOS 10.14.6

6.2.0 - Windows 10