For my Instrustar scope they quote it at 20MHz BW but i measured 15MHz, a little different not too bad i guess,
You must have gotten a real good one. I picked one of these a few years back for shits and giggles because it was the absolute cheapest 20MHz (advertissed) scope. I think I paid $55 CDN for it and was going to use it on my vintage computer bench. However, mine measures a lot worse than yours.
The first shot is a 500KHz reference @ 5V amplitude. The cursors are set for -3dB.
The 2nd shot is at 10MHz. You can see it is well below the -3dB point. This is barely an 9MHz scope
As a backup, the 3rd shot is a 2ns rise time oscillator. You can see being as generous as I can the rise time on the Instrustar is 46ns, giving us a bandwidth of about 8.7MHz. Which agrees with the measurement above.
Not a stellar performer. And the bandwidth is just the beginning of the disappointment. Definitely should be added to the "Do not buy!!!!!!" category.