How to measure real time bps?

Well, you have to understand how the underlying technologies work.    

Using SNMP Solarwinds will periodically poll the device for byte counters on each monitored interface.  Usually this period is every 9 minutes, not exactly real time, but doesn't overload your polling engine either.   You can adjust this to be faster or slower on a global basis, not really recommended though, or you can adjust it on a "per interface" basis also.  For a smallish number of interfaces this wouldn't be too bad, however - most web pages don't refresh all that quickly either.   ie: if you have SW polling the interface every 1 minute, but your webpage refreshes every 5 minutes...

Netflow, which is what NTA utilizes, works very differently.   The device pushes the data to the Netflow receiver every so often.   Some devices might wait until a "flow" is finished, which is NOT recommended.   Most recommendations say to have it push this data every minute, or 45 seconds after the flow finished.   Once again, not real time, but closer.  You get traffic types and such too, which is nice, rather than simple byte counters.

There is a tool called "Engineers Toolkit" which people used to set up outside of Solarwinds, although there is a way to integrate portions of it now, where you would use the SNMP polling on a much faster interval.   Not 100% sure if you could integrate the resources from the web based Engineers toolkit into a NPM dashboard.

The latest version of Solarwinds, 2019.4, did change things a bit.   If you are on this version and go into Interface Details on a given interface, the top graph on the right hand side, now purports to be "Realtime" data.  Not 100% sure what they're doing here, I'd suspect they're doing a bit of what I said Engineers toolkit did for an interface that is brought up.  If you click through on this resource it takes you to PerfStack.

If you edit the resource you'll see it's called "Percent Utilization - Real Time Data".  The problem would be getting it to load for multiple different interfaces on the same page.   I've tried doing stuff like that in the past, but with no luck.   Maybe you can create an actual perfstack resource for each and load that, might be a bit of work to keep it updated though...