Do cloud vendors slow traffic to other cloud vendors?

Do you mean, do cloud vendors intentionally use network bandwidth shaping to make it harder for customers to egress data to another provider?

I don't think so. All of my experience has shown that the main clouds have the best internet connections, and the best connections between their networks. Cloud providers normally use economic disincentives (ingress free, egress charges) instead.

None of the big name VPS providers have done this to my knowledge and I make heavy use of nuttcp/iperf3/iftop and lftp multi-threaded rsync-like tool on all my instances everywhere. If I use a shared vs dedicated cpu instance I sometimes get short bursts of slowness but I expect that on shared instances and the cause is obvious in vmstat.

There was a time and may still be where small AWS accounts had some fixed rate limiting on small EC2 instances but I have no idea if they still do this.

I previously worked for a SaaS provider who had the fastest API for both ingress and egress in our space. The egress often made migrating off far faster than when we migrated competitors in, but we never got in the way of egress speeds.