Sometimes there is a bit of time switching between emails. Has anyone else experienced this?
Granted, it does tend to be when those emails you are switching to are larger than the usual but I just wanted to stress the importance of closely monitoring and testing this.
This was a major issue in Mailbird when they released and 10+ years on, is still a major problem. It caused a lot of users to switch to another application.
Maintaining a quick UI should be the number one priority.
What you’re describing sounds like a symptom of this:
…or this…
Can you confirm whether either of those are related?
Meanwhile…
Everyone has a different idea about “number one priority”. Arguably, getting to one’s email accounts is a common candidate for number one. 
Any posts about slowness when toggling between large emails I would say is probably the same as what I am seeing yes.
Haha, yes, of course being able to access the emails is key. My comment really was just to point out that I have abandoned many an email application in the past due to sluggish performance as I am sure many others have.
I do not have the mailboxes set up in Mailspring as of yet but some of our email accounts have hundreds of thousands of emails in. To be honest, I probably won’t bring them into Mailspring anyway as we route them through our company ticket helpdesk system but there are probably other people out there with insane inboxes - unsure how Mailspring would handle that - the mailbox I am using in Mailspring currently though I try to keep the inbox below 500 - it’s just some of the emails contain 30+ replies so increase in size and this where I am seeing the issue.