Edit ~/.kodi/userdata/advancedsettings.xml. The file won’t exist yet.
Add
<advancedsettings>
<cache>
<buffermode>1</buffermode>
<memorysize>52488000</memorysize>
<readfactor>5.0</readfactor>
</cache>
</advancedsettings>
This will be included by default in the next update