During simulation, LTspice usually uses a compressed binary file format that allows additional simulation data to be appended without modifying the rest of the file. But once the simulation is completed, this file format can be slow to access for the purposes of adding a single new plot trace from the file.
To reduce this time, you can convert the file to an alternative, Fast Access, format. This format can only be done after the simulation is completed when no new data will be added to the file. But once the file is converted to this format, the load time of a new traces will be reduced typically by a factor equal to the number of data traces that have been saved in the file. For example, if you have a 5GB file with 2000 data traces, it might take 4min to add a new trace. But after you convert it to Fast Access format, this four minute load time would be reduced to a single second. This makes cross probing large circuits with huge simulation data files interactive. The exact time it takes to load a trace from a Fast Access format file will depend more on the amount of physical memory you have than your hard disk speed.
To convert a waveform window to Fast Access format, make the waveform window the active window and execute menu command =>Files=>Convert to Fast Access. The conversion process will require an amount of free disk space equal to the file size to be converted, but the converted file will be only 11 bytes larger than the original file.
The conversion process can take a long time and use up to one quarter of your physical memory. In fact, it can take more time to convert the file to Fast Access format then was required for the initial simulation. The exact time the conversion requires will depend on such factors as the state of the hard disk fragmentation and the amount of physical memory you have. During conversion, you may find your machine is not very responsive to your mouse and keyboard. It is possible to convert files in a batch command with the following command line syntax:
scad3.exe -FastAccess <file>
Where <file> is the name of the .raw file you wish to convert to Fast Access format.
This format is only supported for real data, not the complex data that comes from a .ac analysis.