I'm sure I've had this before, but I can't think how/if I fixed it. Yes I can.

I've imported a song in MusicXML and the song tempo is set, but at present it's too quick for me, so I tried to slow it down by changing the tempo in the box above the chord sheet. I tried also via song form. The value for tempo changes to the value I set, but when I generate and play, the tempo has not actually changed and after the first bar the tempo number in the box returns to the imported value.

I've looked at step edit melody ... nothing there, I've looked in piano roll ... nothing there.

Ah, I've just had a thought ... OK, it's hidden away in the bar settings for chorus 1, so it's one of those that don't show a red line. I presume this is a function of the import as I never set it myself.

Now that you mention it, I’ve had to remove an embedded tempo at measure one. I never noticed if it had the red line or not. If it doesn’t, I think it should.

I've never imported an XML type song so I don't know if this setting is applicable but if audio is involved, there's also a Master Tempo setting button that's seen in the attached screen shot.

AFAICS there's no red line if the setting is on a bar in only one chorus.
A red line would definitely help as at least one knows to look.
Someone some time back regarding a similar (maybe the same) problem suggested F5 and then clicking [Print Summary] to get a complete list of bar changes. That's what I did, before selecting 1st chorus bar 1 and then changing the tempo to 0.
One of BIAB's many quirks.

Charlie - I'll try to remember that tip, it could be useful. Thanks.

There's a risk of confusion here unless there's some differentiation between "this bar on some chorus somewhere" and "this bar on all choruses".
Maybe a partial or full line? The present situation is resolvable easiliy enough, but rather arcane.

I was just watching a video on QEMU on Linux it's suppose to be a lot faster than VirtualBox.

I haven't used QEMU for quite some time. I may give it a try.

Edit: QEMU seems to have become several different flavours. I'll need to explore.