I started experiencing the same issues as the OP today but on a smaller scale with a smaller project (1 song; 32 tracks and around 110 FX). When clicking on the ruler or in the arrange window I had to wait for a second or a few more for it to change positions and when clicking on the item it worked normally (no lag). When pressing play, the playback also started unusually late. Some of my custom actions utilized playback actions and those caused even slower response times (waiting for 5-10 seconds for one of those actions to respond). My laptop is IMO a strong machine (32Gb of RAM and an i7 8th gen processor).
I found this thread and then after reading
haveigonemental's post I remembered I
changed my computer's performance settings yesterday for a recording project. When I purchased my laptop it had a preinstalled utility called "Control Center" which allows me to switch between some performance settings presets and I switched to the "quiet" one yesterday for that recording session - the laptop had to be in the same room as the mic. Now
when I switched back to the "performance" setting the problem disappeared.
Thank you haveigonemental for writing your post. I hope it helps others too. It has helped me resolve my issue (at least for the time being
).