FastDosBox is an enhanced fork of DOSBox, an open source x86 PC emulator. Based on the SVN source code of DOSBox, Users report that FastDosBox is 250% faster than the version of DOSBox on apt-get. The performance of FastDosBox is said to be comparable with that of a 486DX20
The ingredients for the "secret sauce" of FastDosBox include a fixed/ported dynrec x86 emulation part, support for dispmanx, and couple of assembler subroutines.
The benchmark used to compare DOSBox and FastDosBox, and which leads to the claim of a 250% gain in speed, is 3DBench, a benchmark often used in the DOSBox community to compare 386-486 Pentium class machines (https://docs.google.com/spreadsheet/ccc?key=0Aigi3Xn2U-_vdG12bmtlQXJNLUNuQjdBV2ZqWnVzQkE#gid=0). Running the benchmark on FastDosBox on a Raspberry Pi gives a result of 12.0 - equivalent to a 486DX20 - whereas running the version of DOSBox 0.74 on the same board gives a result of 5.