I have the same problem, noticed it a few weeks ago, looking to resolve it before ma next gig Sat. When first booting Arena up, it's fine if I flick through columns, then within a couple of minutes and waiting for a column to play for at least 10ish seconds it'll 'pause' between column shifts. It's really noticeable. And uncool.
Machine specs:
macOS Catalina v 10.15.5
MacBook Pro (15-inch 2019)
Processor 2.4 GHz 8-Core Intel Core i9
Memory 32 GB 2400 MHz DDR4
Graphics Radeon Pro 560X 4 GB
Intel UHD Graphics 630 1536 MB
Arena 7.2.1 rev70674 July 2020