This seems to be the case. I use top to check CPU usage, and in the idle status, texmacs.bin comsumes about 3%, even more significant than the OP.
I use gdb to attach texmacs.bin, and get backtrace from time to time, which looks like
(gdb) bt
#0  0x00007fab39c6f3ff in poll () from /usr/lib/x86_64-linux-gnu/libc.so.6
#1  0x00007fab3928b0ae in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007fab3928b1cf in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007fab3a55c51f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#4  0x00007fab3a50398b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#5  0x00007fab3a50bc00 in QCoreApplication::exec() () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#6  0x000055e9b4db3761 in ?? ()
#7  0x000055e9b5c00b00 in ?? ()
I did not build texmacs myself therefore there is no debug symbol for ??.