Version 3.783 did it for me!!!
BTT is now swift as it was before !
And I don't seem to be having the beachball / unresponsiveness that Shnub is experiencing.
I am very curious to know what the actual issue was 
The apparent freezing was the default behavior for a long time - with one of the versions around 3.75xx I made changes (for other reasons) that also prevented this. But the bug you described (with the repeated trigger before delay has finished) was caused by this change. After more investigation I figured this can also cause actions to be executed in the wrong order.
3.783 makes BTT pause (freeze) only if this situation occurs (repeated trigger within the delay) to prevent the issues described. Also I think it doesn't happen on the latest macOS versions due to changes in the OS.
3.784 might keep the BTT UI more or less responsive while maintaining the bugfixes. Let's see.
I'm not seeing 3.784 yet, will try it when I do.
Is BTT predicated on the assumption that all actions will be run sequentially? I always thought that it was entirely feasible to run them in parallel, but I never had the need (excepting scripts which can be run asynchronously) so never actually tried it until now.
PS:
That's not what I was seeing. Rather a delay running in one action would completely block all other actions (i.e. there were no repeated triggers!). That's what seems wrong to me.
Yes, BTT will only run actions sequentially, any parallel running would/could cause issues. This has various technical reasons, one of them is that Apple Script can also only be run sequentially (same goes for the Apple Shortcuts app as far as I know, but this might have changed)
Sorry - I forgot to push the 3.784, will be online in a few minutes.
I see. But how did 3.756 manage to circumvent this then? With that version, I can run a delay in one action and trigger any other action while the delay is running. For example, I can run a dummy action containing just one 1-minute trigger and while that is running (i.e. delaying), I can trigger whatever else I want (e.g. my custom Mission Control action or "Bring Window Under Cursor to Front").
With 3.784, BTT just crashes if I trigger anything while a delay is running.
With 3.756 It was random, would work for some actions, but would screw up other actions and break the order of things. If you had two triggers, one that outputs ABC and one that outputs DEF it could have resulted in ADEBFC in that version.,
Especially for action sequences that interact with the UI (e.g. clicking elements etc.) it's crucial that no other action sequence can do things in-between.
Weird, I haven't seen crashes here, but I'll revert this to the 3.783 version then.
Sorry, that's what I meant. Running another action sequence while one is currently being executed.