It's not perfect, but it seems to work:
Configure button > (select the virtual drive or the shortcut) > General tab > Debug flags: "fps"
Then go back to the main window > Select the shortcut > Debug (in the side panel)
And the debugger window will start filling with messages like
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.26fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.26fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.18fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.16fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.12fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.24fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.24fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.20fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.18fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.26fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.28fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.22fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.18fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.22fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.16fps
trace:fps:swapchain_gl_present 0x1b0628 @ approx 30.05fps
...