In Scrum, you should not see the distinction between "developer" and "QA tester". As of November 2020, the Scrum Guide says:
Within a Scrum Team, there are no sub-teams or hierarchies. It is a cohesive unit of professionals focused on one objective at a time, the Product Goal.
A Scrum Team has one Product Owner, one Scrum Master, and Developers. Developers include all of the skills necessary to create a usable product Increment every Sprint, which may or may not include various specialist roles. The team, as a whole, is cross-functional, but individuals also strive to be cross-functional. People should generally be able to contribute to other aspects of the work as well.
If time testing is the bottleneck, the QA tester should work with the developers to improve their testing ability. If the developers are working on testing, then the workload can be shared. There are lots of ways to help share the workload. Perhaps the developers write automated tests to reduce the likelihood of regressions, meaning the QA test specialist can focus on testing the new functionality and exploratory testing. Perhaps the developers learn manual testing techniques and can share the burden by testing each other's work. The team may have other ideas.
If the team has reached its limit for what it, as a whole, can get Done in the Sprint, use the opportunity to increase the skills of the team so there are more people to be able to take on a wider variety of work and the team's capacity, as a whole, will increase.